High quality monaural phonograph records containing little or no inorganic fillers have long been made of compositions comprising synthetic resins such as copolymers of vinyl chloride-vinyl acetate or polystyrene. It has also been conventional to include certain additives in small amounts in the compositions to improve the record molding process or to impart particular desirable properties to the record. Typical additives are stabilizers to inhibit decomposition of the resin during the mixing and molding operations, plasticizers to enhance the molding properties of the resin, solid antistatic agents to prevent collection of dust in the sound grooves of the record, carbon black to impart a uniform coloration and sheen to the record, and mold release agents to inhibit damage to the record when it is released from the mold.
With the advent of 2-channel stero recordings, some changes were made in certain additives such as stabilizers, but the resin systems remained essentially similar to those used in the older monaural recordings.
In the case of both the older monaural system records and the newer 2-channel stereo records, the process of compounding the ingredients is essentially the same. All of the ingredients are introduced at the same time into a mixing apparatus (usually a Banbury mixer) and thoroughly dispersed until they become a uniform mixture. Due to internal friction within the composition, during the mixing process the temperature rises to the fusion point of the mixture and the mixture is thus converted to a melt which is later either sheeted, cooled and ground into granules, or handled as a molten material in an extrusion apparatus which feeds an automatic record press.
Now, however, 4-channel stereo systems have been developed and these have placed much more stringent demands on the recording material of the phonograph record discs. The discrete 4-channel record is based upon the success of reliably reproducing signals up to 45 kHz from the recorded groove. Two-channel stereo records, on the other hand, require reproducing a signal bandwidth approaching but not exceeding 15 kHz. The higher the frequency of the actual recorded signal, the greater must be the number of recorded undulations per unit of groove length.
A 2-channel stereo groove has undulations of 2 different bandwidths molded into the opposite sloping walls of the record groove. The pickup stylus, riding in the groove, vibrates at both of these frequencies and the sound signal reproducing system separates these 2 signals and feeds them to separate amplifiers and speakers.
In a 4-channel system, 2 different fm carriers are superimposed on the groove walls in addition to the undulations required for a 2-channel system. Each of these carriers is independently frequency modulated by a signal source associated with a particular carrier.
The higher bandwidth requirement of 4-channel stereo records results in a requirement for reproducible signal density per unit of groove wall area, 3 times as great as that required for 2-channel stereo records and this requirement is superimposed upon the modulation of the basic stereo groove which already requires a reproducible bandwidth of 15 kHz. Thus, the signal density requirement in a discrete 4-channel record groove is 4 times that for a 2-channel stereo groove.
There are also other requirements for a satisfactory 4-channel stereo phonograph record that are not present, or at least not present to the same extent, in ordinary monaural or 2-channel stereo records. In the 4-channel system records, the record surface must not only have the best obtainable long-wearing properties, but the surface must also wear in a smooth manner instead of in a porous granular manner previously experienced. If the surface does not have the required wearing properties, the fm carrier modulations are degraded and very small particles of record material become broken off the groove walls.
An additional problem arises if the broken away particles of record material accumulate in the bottom of the record groove. Then the pickup stylus starts to ride higher in the groove and does not make full contact with the groove walls. When this happens, the stylus contacts a smaller area of wall, pressure on the wall becomes correspondingly greater, and record wear becomes more rapid. When record wear becomes more rapid, sound reproduction deteriorates more rapidly.
Using the older system of mixing all of the record composition ingredients together at one time, the present inventor found that he could not make commercially acceptable 4-channel stereo records, regardless of the composition variations that he tried. However, with the present invention, which includes an improved mixing process and also includes a proper selection of ingredients and proportions of ingredients, commercially acceptable 4-channel stereo records have now been achieved.